Energetics and Mechanism for H2S Adsorption by Ceria-Lanthanide Mixed Oxides: Implications for the Desulfurization of Biomass Gasifier Effluents

Density functional theory (DFT) and ab initio thermodynamics are used to calculate the free energies of H2S adsorption and dissociation on CeO2(111) and CeO2(111) I doped with La or Th. Experimental sulfur capacities are reported for La- and Tb-doped CeO2 adsorbents for comparison with computed energetics. The DFT-based free energies of H2S adsorption, dissociation, and oxygen vacancy formation are the sulfur adsorption process occurs via H2S adsorption and dissociation over substoichiometric oxygen vacancies, and is rate limited by a strongly endergonic molecular adsorption of H2S. Sulfur incorporation is only favorable if multiple adjacent oxygen vacancies are present to provide the flexibility required to accommodate the larger coordination shell of sulfur atoms. The thermodynamics of the overall H2S adsorption process do not correlate with oxygen vacancy formation energy, implying that the optimization of ceria-based sulfur sorbents cannot be achieved by tailoring composition to maximize the exergonicity of vacancy formation. The rate-limiting step for H2S adsorption and dissociation involves reaching the transition state for dissociation of the first S-H bond to form SH* + H* (>2.23 eV over each surface), and this rate is highest over ceria-lanthana (>ceria-terbia > ceria). This is the same order as the experimental sulfur capacities, if these capacities are compared on a similar molar (Ce/La vs Ce/Tb) basis. Agreement with the experimental capacities suggests that the actual sulfur capacities of ceria-based mixed oxides are largely determined by the kinetics of the H2S dissociation.